Among various wiring boards used in electronic equipment, there is an increasing demand for a flexible wiring board because of its flexibility, small thickness, light weight, etc. Furthermore, the use of the flexible wiring board as a base material of a semiconductor package or a package for a liquid-crystal module also is increasing. The flexible wiring board includes a polyimide film as an electrically insulating base material and a copper wiring formed on the polyimide film.
Among various methods for producing the flexible wiring board, a sputtering-plating method is attracting attention because minute wirings can be formed easily by this method, for example. According to the sputtering-plating method, the flexible wiring board is produced in the following manner. First, a thin layer of chromium, a nickel-chromium alloy, or the like, which serves to adhere polyimide and copper, is formed on the polyimide film as a base material. Then, a copper layer is formed by electrolytic plating on a portion where a wiring circuit is to be formed. Thereafter, a portion of the thin layer that is not covered with the copper layer is removed by etching, thus forming a copper wiring (see JP 2000-252625 A).
The removal of the thin layer generally is carried out using an aqueous solution that contains ferric chloride as a main component.
However, the conventional aqueous solution that contains ferric chloride as a main component has a problem in that it may cause too much dissolution of copper.